Load balancing servers have heretofore been evolved for the purpose of handling huge traffic by sharing traffic which would otherwise have to be handled by a Web server across multiple Web servers having equivalent functions. Load balancing is now being developed, extending its scope to cover a variety of devices and protocols involving routers, mail servers, VPN gateways, etc., besides Web servers.
Among these network elements, a server that realizes load balancing across SIP servers is included. Taking account of specifications specific to a Session Initiation Protocol (SIP) (see RFC 3261), this load balancing server is provided with a function of allocating traffic of a persistent dialog to one server. A typical method that is used to implement the function of allocating traffic of a persistent dialog to one server (persistence function) is to refer to a CALL-ID value included in a SIP header and allocate messages having the same CALL-ID value to one server.
There also exists a load balancing server for traffic of encrypted communication such as the one that supports Hypertext Transfer Protocol Security (HTTPS) encrypted communication using a Secure Socket Layer (SSL). As a method for realizing a load balancing server, there exists a method in which the load balancing server notifies a terminal of the address of an assigned server and instructs the terminal to reconnect to the assigned server specified by the load balancing server (e.g., see JP-A No. 334012/2002).